The present invention relates generally to communications systems employing forward error correction (FEC) coding to reduce transmission errors, and more particularly, to an improved and flexible trellis coded modulation (TCM) decoder and system.
U.S. Pat. No. 5,363,408 entitled xe2x80x9cMode Selective Quadrature Amplitude Modulation Communication Systemxe2x80x9d discloses a quadrature amplitude modulation (QAM) communication system in which data can be communicated in any one of a plurality of QAM modes, such as 16QAM, 32QAM, and 64QAM. A receiver detects the particular QAM mode transmitted on a trial and error basis, by attempting to decode the received data using different QAM modes until a synchronization condition is detected. The synchronization condition can require that a plurality of different synchronization tests be met. In a specific embodiment, a first synchronization test is met when a renormalization rate of a trellis decoder is below a threshold value. A second synchronization test is met when a first synchronization word is detected in the received data. A third and final synchronization test is met when a second synchronization word is detected in the received data. In order to reduce the cost of the receiver, most of the QAM mode dependent components are implemented using look-up tables stored in programmable read only memories (PROMs).
U.S. Pat. No. 5,363,408 discloses a decoder having a look-up table structure that is somewhat similar to that of the present invention. The system of U.S. Pat. No. 5,363,408 requires a custom Viterbi decoder chip and has relatively large RAM look-up tables. The system of U.S. Pat. No. 5,363,408 passes path metrics (16 bits) to the custom Viterbi decoder chip, which is not done in the present invention. The look-up table of the system of U.S. Pat. No. 5,363,408 is used to make hard decisions on the uncoded bits, which is not done in the present invention.
It would therefore be advantageous to have a trellis coded modulation decoder that makes use of a commercially available Viterbi decoder chip and requires smaller RAM look-up tables.
U.S. Pat. No. 5,363,408 discloses the use of a trellis coded modulation decoder that is limited to 16QAM, 64QAM, and 256QAM modulation schemes. U.S. Pat. No. 5,363,408 does not disclose or suggest a trellis coded modulation decoder that allows a choice of modulation schemes that may be tailored to a communication channel.
Therefore, it would be advantageous to have a flexible trellis coded modulation decoder and system having minimum hardware complexity that is not limited to 16QAM, 64QAM, and 256QAM modulation schemes, but allows a choice of modulation schemes tailored to the communication channel.
The present invention provides for a trellis coded modulation system for use with a plurality of QAM modes, including but not limited to, 16QAM, 64QAM, and 256QAM. The present invention allows a choice of modulation scheme that is tailored to the communication channel using a decoder having minimum hardware complexity.
Look-up tables in the encoder and decoder allow the use of a single encoder/decoder to map trellis coded modulation to/from multiple modulation constellations. Whereas the mode selective QAM system disclosed in U.S. Pat. No. 5,363,408 uses a similar look-up table structure, the application disclosed in this patent is different from that of the present invention.
The size of the RAM look-up tables used in the present decoder is minimized. Instead of passing path metrics (16 bits) to a custom Viterbi decoder chip, as is done with the mode selective QAM system of U.S. Pat. No. 5,363,408, recognizable sequences (6 bits) are passed to an off-the-shelf Viterbi decoder chip in the present decoder.
The look-up table of the system of U.S. Pat. No. 5,363,408 is used to make hard decisions on uncoded bits. In the present decoder, an input look-up table provides minimal information about which region the symbol was found, and an output look-up table processes the resulting information appropriately. The efficient use of look-up tables in the present decoder greatly reduces complexity and the required RAM size. Use of an off-the-shelf Viterbi decoder in the decoder reduces complexity and cost.
Thus, the present invention uses fast look-up tables for speed, simplicity, and flexibility. The decoder is capable of operating with 16QAM, 64QAM, and 256QAM modulation, along with other modulation formats such as 32CROSS modulation, for example. The concepts of the present invention may easily be used with higher order QAM constellations, CROSS constellations, and any arbitrary digital signaling constellations. Accordingly, the present invention is not limited to only QAM constellations.